PRINCETON, N.J., May 14 /PRNewswire-FirstCall/ -- Pharmasset, Inc.
(Nasdaq: VRUS) has commenced dosing two additional cohorts of a 4-week
Phase 1 study of R7128 in combination with Pegasys(R) (peginterferon
alfa-2a) plus Copegus(R) (ribavirin) in both treatment-naive and
treatment-experienced patients chronically-infected with hepatitis C virus
(HCV) genotypes 1, 2 and 3. R7128, a prodrug of PSI-6130, is a nucleoside
analogue polymerase inhibitor of HCV that is being developed through
Pharmasset's collaboration with Roche.

The purpose of this 4-week study is to evaluate the safety,
tolerability, pharmacokinetics and antiviral activity of R7128 in the
clinically-relevant setting of combination therapy for chronic HCV
infection. The previously planned Cohort 3 will continue dose-exploration
with administration of R7128 1000mg twice-daily (BID) in treatment-naive
patients with HCV genotype 1. Cohort 4 will evaluate R7128 1500mg BID
administered in combination with Pegasys plus Copegus in
treatment-experienced patients with genotypes 2 or 3 who did not achieve a
sustained virologic response (SVR) with previous interferon-based therapy.

There will be 25 patients in each dose cohort with 20 patients
randomized to receive R7128 and 5 patients randomized to receive placebo.
After completing 4 weeks of the triple combination regimen and a follow-up
period of 4 weeks of Pegasys plus Copegus, patients will receive an
additional 16 to 40 weeks of open-label dosing of Pegasys plus Copegus
under a separate protocol to complete the standard of care regimen for each
genotype. Preliminary safety and antiviral activity data from the 4-week
combination treatment period are anticipated during the third quarter of
2008.

"Based on our pharmacokinetic modeling, we believe that the R7128
1000mg BID combination dosing regimen in Cohort 3 may be able to achieve
similar antiviral responses as were demonstrated with R7128 1500mg BID in
Cohort 2," stated Dr. Michelle Berrey, Pharmasset's Chief Medical Officer.
"Cohort 4 will be the first administration of R7128 in patients with HCV
genotypes 2 or 3, and thus will serve as proof-of-concept for this
population who we believe represent an area of great unmet medical need. If
R7128 shows activity in HCV genotype 2 or 3 patients, this would
demonstrate a distinct clinical attribute of nucleoside polymerase
inhibitors."

R7128 is being developed for the treatment of chronic HCV infection.
R7128 is a prodrug of PSI-6130, a cytidine nucleoside analog inhibitor of
HCV RNA polymerase. A prodrug is a chemically modified form of a molecule
designed to enhance the absorption, distribution and metabolic properties
of that molecule. Results from a Phase 1 oral single ascending dose study
of R7128 in 46 healthy male volunteers showed that R7128 was generally well
tolerated with no serious adverse events in doses up to 9000 mg.

In a Phase 1 study, R7128 demonstrated potent, dose-dependent antiviral
activity across four genotype 1 prior treatment-failure patient cohorts
(n=40) receiving 750 mg or 1500 mg administered either once-daily or
twice-daily for 14 days as monotherapy. The greatest mean decrease in HCV
RNA from baseline was demonstrated in the patient cohort that received 1500
mg twice-daily, the highest dose of R7128 administered in the study. These
patients demonstrated a mean 2.7 log10 IU/mL (>99%) decrease in HCV RNA.
There was no evidence of the development of viral resistance in any dose
cohort after 14 days of dosing.

Hepatitis C is a blood-borne infectious disease of the liver and is a
leading cause of chronic liver disease and liver transplants. The WHO
estimates that nearly 180 million people worldwide, or approximately 3% of
the world's population, are infected with hepatitis C virus (HCV). The CDC
has reported that almost four million people in the United States have been
infected with HCV, of whom 2.7 million are chronically infected.

About Pharmasset

Pharmasset is a clinical-stage pharmaceutical company committed to
discovering, developing and commercializing novel drugs to treat viral
infections. Pharmasset's primary focus is on the development of oral
therapeutics for the treatment of hepatitis B virus (HBV), hepatitis C
virus (HCV) and human immunodeficiency virus (HIV).

Pharmasset is currently developing three product candidates. Clevudine,
for the treatment of chronic HBV infection, is enrolling Phase 3 clinical
trials for registration in North, Central and South America and Europe.
Clevudine is already approved for HBV in South Korea and marketed by
Bukwang Pharmaceuticals in South Korea under the brand name Levovir. R7128,
an oral treatment for chronic HCV infection, is in a 4-week Phase 1
clinical trial in combination with Pegasys(R) plus Copegus(R) through a
strategic collaboration with Roche. Racivir, which is being developed for
the treatment of HIV in combination with other approved HIV drugs, has
completed a Phase 2 clinical trial.
Pegasys(R) and Copegus(R) are registered trademarks of Roche.

Pharmasset "Safe Harbor" Statement under the Private Securities
Litigation Reform Act of 1995: Statements in this press release regarding
our business that are not historical facts are "forward-looking statements"
that involve risks and uncertainties, including without limitation the risk
that adverse events could cause the cessation or delay of any of the
ongoing or planned clinical trials (including, without limitation, the two
additional cohorts of our ongoing Phase 1 clinical trial of R7128) and/or
our development of our product candidates, the risk that R7128 shows no
activity in HCV genotype 2 or 3 patients, the risk that we cannot enroll
enough patients for the Phase 3 registration clinical trial for clevudine,
the risk that our collaboration with Roche will not continue or will not be
successful and the risk that any one or more of our product candidates will
not be successfully developed and commercialized. For a discussion of these
risks and uncertainties, any of which could cause our actual results to
differ from those contained in the forward-looking statements, see the
section of our Annual Report on Form 10-K for the fiscal year ended
September 30, 2007 filed with the Securities and Exchange Commission
entitled "Risk Factors" and discussions of potential risks and
uncertainties in our subsequent filings with the Securities and Exchange
Commission.

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